Interstitial pneumonia is a heterogenous group of pulmonary disorders that produce pulmonary fibrosis in middle-aged to older horses (average age 13 yr). This condition has been hypothesized to result from multiple etiologies, including toxins and idiosyncratic reactions to common pathogens. Affected horses present with tachycardia, tachypnea, respiratory difficulty at rest, lethargy, fever, and weight loss. Thoracic radiography reveals a diffuse, severe, nodular interstitial pattern. At necropsy, the lung fails to collapse, and rib impressions are present on the parenchymal pleural surface. Histopathologic evaluation of biopsy or postmortem samples identifies pulmonary fibrosis characterized by expansion of the alveolar parenchyma with collagen and intraluminal accumulation of neutrophils and macrophages. Recently, investigators identified intranuclear inclusion bodies within alveolar macrophages in a population of horses with interstitial pneumonia and isolated equine herpesvirus 5 from affected horses. The form of interstitial pneumonia has been termed multinodular pulmonary fibrosis. Interstitial pneumonia with pulmonary fibrosis is considered minimally responsive to aggressive therapy with anti-inflammatory medications, antibiotics, and corticosteroids. Although the prognosis for these cases remains guarded, administration of acyclovir (20 mg/kg, tid for 6 wk) may improve survival in horses with equine herpesvirus 5 infection.

Last full review/revision March 2012 by Bonnie R. Rush, DVM, MS, DACVIM